In many radio frequency, RF, applications power amplifiers are employed to amplify high frequency signals. Because the RF amplifiers are biased to provide substantially high output power, they exhibit nonlinear responses to some degree. Consequently, in response to an increase in the input signal power, such RF amplifiers generate intermodulation IM components, which may have frequencies that are outside a desired frequency band.
One solution to eliminate the consequences of the nonlinear response of the amplifier is to employ multiple amplifiers each configured to amplify a predetermined carrier signal. For example, in a mobile communications environment, the base station sends multiple carrier signals in accordance with time division multiple access (TDMA) modulation scheme, or in accordance with code division multiple access (CDMA) modulation scheme. Each carrier frequency in TDMA corresponds to one of the users in a specifiable cell. Each pseudo-code in CDMA corresponds to one user. Because the base station has to communicate with many users in the corresponding cell, the intermodulation IM components increase with the number of the users. Thus, the use of a separate amplifier for each carrier signal substantially eliminates the generation of intermodulation IM components. However, this approach is costly and may not be commercially feasible in many applications.
Another approach is to employ an analog linearizer, such as 10 as illustrated in FIG. 1. The operation of linearizer in conjunction with an exemplary two-tone signal is described hereinafter. Basically, a radio frequency signal represented by frequency components 22 is fed to a power amplifier 12. Amplifier 12 generates additional intermodulation IM frequency components 24 because of its nonlinear response characteristics. Signal components 22' correspond to an amplified version of signal components 22. The function of linearizer 10 is to substantially eliminate frequency components 24, as explained in more detail below.
Linearizer 10 includes a signal cancellation circuit 26 coupled to an error cancellation circuit 28. Signal cancellation circuit 28 has an upper branch that includes power amplifier 12, and a lower branch that provides the input signal of the linearizer to an input port of an adder 16. The other input port of adder 16 is configured to receive the output signal generated by power amplifier 12, via an attenuator 14. As a result, the output port of adder 16 provides signal components 24', which correspond to the attenuated version of intermodulation IM frequency components 24.
Error cancellation circuit 28 also includes an upper branch that is configured to provide the output signal generated by amplifier 12 to an adder 20. The lower branch of error cancellation circuit 28 includes an amplifier 18, which is configured to receive the attenuated intermodulation components 24'. Amplifier 18 generates an amplified version of signal 24' which is substantially equal to intermodulation component 24. As a result, the output port of adder 20 provides signal components 22' without the distortion caused by the amplifier
The feedforward linearizer described in FIG. 1 has some disadvantages. For example, because it is based on analog circuitry, it cannot quickly adapt to signal changes. Other design solutions to accomplish linearization include pre-distortion linearizer arrangement and feed-back linearizer arrangement. However these designs do not provide acceptable processing. Furthermore, feed-back linearizers often lead to signal instability.
In order to avoid the problems associated with analog feedforward linearizers, feedforward linearizer employing digital signal processing techniques have been proposed which adaptively adjust some control parameters (or adjustment signal) in the circuit to accommodate the change in signal and environment. However, slow-convergence on the adaptation of the adjustment signals in the proposed solutions, restricts the effectiveness of such feedforward linearizers.
Thus, there is a need for a feedforward linearizer that employs digital signal processing techniques and that provides effective suppression of intermodulation components and fast convergence.